Spot welding is used to join parts made of sheet metal. In this process two copper electrodes have replaceable caps that press the two sheets together. Current is then passed through the electrodes and the sheet metal joint by applying a voltage across the electrodes. Due to the passing of current, resistance heating takes place in the sheet metal joint. If the current and the duration for which the current is applied is adequate to cause melting of the joint, the two sheets can be bonded together. Such a bond is referred to as a spot weld.
The variables of such joints are stack thickness, the number of interfaces and the governing metal thickness. The governing metal thickness is the thickness of the thinnest sheet in a two sheet joint. The joint variables are used to determine the process parameters, which are then programmed into a controller. The controller is thus capable of producing acceptable welds. The process parameters include current, force, time and cap size. The cap size is specified by the tip diameter.
In a typical production run in the plant, the electrode caps are changed at regular intervals since the cap tips degrade after producing a number of welds. Typically caps are changed every half a shift, or about every four hours. In a typical half shift, a given set of caps can produce as many as 6000 spot welds. The cap tip area is the area that is in contact with the sheet metal when the proper interjoint force is applied. At the start of a production run, the initial cap tip diameter may be 4.8 mm. By the end of the run the diameter may increase to 10 mm. This increase in diameter occurs due to wear and forging of the cap tips. An increase in surface area of the cap tip, reduces the interjoint pressure and current density. This change has a negative effect on the process and resulting spot weld.